# DIY 75ohm to 50ohm impedance matching circuit to work at UHF frequencies around 470MHz to 600Mhz

I have a transmitter circuit that has 75ohm impedance with the F-type connector. I need to connect this device to a 50ohm impedance power splitter/combiner with SMA type connector. The transmitter device is operated at UHF frequencies around 470-600Mhz (nothing over the air, all testing is done via cables). Is there a DIY solution to build a impedance matching circuit which can work in the above mentioned scenario ?

Edit : The transmitter's output power is around 26dBm to 30dBm, which is very low power.

• I work on an IC transmitting at 60 GHz and it outputs only 11 dBm at maximum power ! We (me and my colleagues) can only dream about transmitting 26 dBm ;-) Commented May 18, 2016 at 7:21
• Let me raise the bar then, the transmitter I have mentioned in the question is permitted by FCC to emit upto 36dBm(4Watts approx.) :) Commented May 18, 2016 at 7:29
• If this problem is due to your wrong splitter, then you have to make sure that coax cables are 50 ohm, as well all the receivers. It's quit uncommon that you have the equipment with mixed impedances. But..if you don't want to loose power I'd suggest you to make a LC matchbox, air autotransformer and few caps, and SWR meter to tune. Commented May 18, 2016 at 9:13

The dodgy answer. There's not going to be a huge return loss between 50ohm and 75ohm connected directly. At these low powers, your transmitter will probably handle the reflection OK, and you will lose some power to the output, but maybe not all much.

The better answer. Use a lambda/4 length (at 530MHz) of 61 ohm transmission line to transform between one impedance and the other. If you don't have any 60ohm line (it is available, and for this very purpose) then you could design a 61 ohm low pass filter (3rd order ought to be sufficient at this bandwidth) with an electrical length of 90 degrees at 530MHz.

The best answer. Design a lowpass filter between unequal impedances, so 50 on one port, 75 on the other.

• I think I will go with the better answer. Sounds the most feasible one right now to me. Commented May 18, 2016 at 13:40

The simplest solution is to accept some signal (power) loss by using a two resistor solution like this:

But a better solution (less signal power loss) would be to use a transformer based impedance adapter. However these are more difficult to make. There are of course also ready-made devices you could use like this one:

• Thanks for the suggestion, but my transmitter is operated with ver low power - around 26dBm to 30dBm, which is why I am afraid of power loss. I will edit the question about the operating power region as well. Commented May 18, 2016 at 7:13
• Then you have little choice but to go for a transformer based solution. But since the resistor based solution is so simple, you could just try it and see if the loss is acceptable to you (sorry, too lazy to calculate the actual loss, my guess would be that it is around 6 dB). Commented May 18, 2016 at 7:18